An electric power steering apparatus (EPS) serves as an apparatus which is equipped with a motor control unit to control a motor. The electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of the motor, applies a driving force of the motor being controlled with an electric power supplied from an inverter to a steering shaft or a rack shaft by means of a transmission mechanism such as gears. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a pulse width modulation (PWM) control. A brushless motor, which has an excellent maintenance performance, is generally used as the motor.
A general configuration of the conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel (handle) 1, is connected to steered wheels 8L and 8R through reduction gears in a reduction section 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 are provided with a torque sensor 10 for detecting a steering torque Ts of the steering wheel 1 and a steering angle sensor 14 for detecting a steering angle 8, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 30 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 30 through an ignition key 11. The control unit 30 calculates a current command value of an assist command (steering assist) based on a steering torque Ts detected by the torque sensor 10 and a vehicle speed Vs detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 for EPS based on a voltage control command value Vref obtained by performing compensation and so on with respect to the current command value.
As well, a steering angle sensor 14 is not indispensable and may not be provided. It is possible to obtain the steering angle from a rotational position sensor such as a resolver connected to the motor 20.
A controller area network (CAN) 40 to receive/send various information of the vehicle is connected to the control unit 30, and it is possible to receive the vehicle speed Vs from the CAN 40. Further, it is also possible to connect a non-CAN 41 receiving/sending a communication, analog/digital signals, a radio wave or the like except with the CAN 40 to the control unit 30.
In such an electric power steering apparatus, the control unit 30 mainly comprises a CPU (including an MCU, an MPU, and a microcomputer and so on), and general functions performed by programs within the CPU are, for example, shown in FIG. 2.
Functions and operations of the control unit 30 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Ts detected by the torque sensor 10 and the vehicle speed Vs detected by the vehicle speed sensor 12 (or from the CAN 40) are inputted into a current command value calculating section 31. The current command value calculating section 31 calculates a current command value Iref1, based on the steering torque Ts and the vehicle speed Vs with reference to an assist map or the like, which is a control target value of a current supplied to the motor 20. The calculated current command value Iref1 is inputted into a current limiting section 33 via an adding section 32A, the current command value Irefm that is limited the maximum current in the current limiting section 33, is inputted into a subtracting section 32B. A deviation I (=Irefm−Im) between the current command value Irefm and a motor current value Im which is fed-back is calculated in the subtracting section 32B, and the deviation I is inputted into a PI-control section 35 for improving a current characteristic of the steering operation. The voltage control command value Vref that the characteristic is improved in the PI-control section 35, is inputted into a PWM-control section 36, and the motor 20 is PWM-driven through an inverter 37 serving as a driving section. The motor current value Im of the motor 20 is detected by a motor current detector 38 and is fed-back to the subtracting section 32B. An FET is used as a driving element in the inverter 37, and the inverter 37 is constituted by a bridge circuit of the FETs.
A compensation signal CM from a compensation signal generating section 34 is added at the adding section 32A. A characteristic compensation of the steering system is performed by adding the compensation signal CM, and then a convergence, an inertia property and so on are improved. The compensation signal generating section 34 adds a self-aligning torque (SAT) 34-3 with an inertia 34-2 at an adding section 34-4, further adds the result of addition performed at the adding section 34-4 with a convergence 34-1 at an adding section 34-5, and then outputs the result of addition performed at the adding section 34-5 as the compensation signal CM.
Even if a motor failure (including an abnormality) occurs in such the electric power steering apparatus, cases that use the motor with multi-system motor windings which has a structure of a continuous motor operation are recently increasing. For example, in the motor with dual-system windings, coils of a stator are divided into two systems (U1 phase to W1 phase and U2 phase to W2 phase). Therefore, even if the failure occurs in one system, the other system can rotate a rotor and successively continue an assist control.
As an example of the motor with multi-system motor windings, a three-phase motor with dual-system motor windings is described with reference to FIGS. 3 and 4.
As shown in FIG. 3, the three-phase motor 200 has a configuration of an SPM (Surface Permanent Magnet) motor that includes a stator 12S having teeth T which are magnetic poles and form slots SL inwardly protruding at an inner periphery, and an eight-pole surface magnet-type rotor 12R which is rotatably disposed opposite to the teeth T at the inner periphery of the stator 12S and are mounted permanent magnets PM on the surface thereof. Here, the number of the teeth T of the stator 12S is set to “phase number×2n” (“n” is an integer which is two or more). For example, in a case of n=2, the motor has a configuration of eight poles and twelve slots.
In the dual-system as shown in FIG. 4, three-phase motor windings L1 of the first system and three-phase motor windings L2 of the second system, which are a poly-phase motor windings that each of the same phase magnetic poles is in phase with the rotor magnets, are wound on the slots SL of the stator 12S. In the three-phase motor windings L1 of the first system, respective one-ends of a U-phase coil L1u, a V-phase coil L1v and a W-phase coil L1w are connected each other so as to form a star-connection. The other ends of the phase coils L1u, L1v and L1w are connected to the motor control unit, and motor driving currents Iu1, Iu1 and Iw1 are individually supplied to the respective coils.
In the phase coils L1u, L1v and L1w, two coil sections Ua1 and Ub1, Val and Vb1 and Wa1 and Wb1 are respectively formed. These coil sections Ua1, Val and Wa1 are, with concentrated windings, wound on the teeth T1, T2 and T3 which are disposed in a clockwise direction. Further, the coil sections Ub1, Vb1 and Wb1 are, with the concentrated windings, wound on the teeth T7, T8 and T9 which are disposed in the clockwise direction being diagonal to the teeth T1, T2 and T3 with respect to the rotor 12R.
Similarly, in the three-phase motor windings L2 of the second system, respective one-ends of a U-phase coil L2u, a V-phase coil L2v and a W-phase coil L2w are connected each other so as to form the star-connection. The other ends of the phase coils L2u, L2v and L2w are connected to the motor control unit, and motor driving currents Iu2, Iv2 and Iw2 are individually supplied to the respective coils.
In the phase coils L2u, L2v and L2w, two coil sections Ua2 and Ub2, Va2 and Vb2 and Wa2 and Wb2 are respectively formed. These coil sections Ua2, Va2 and Wa2 which are, with the concentrated windings, wound on the teeth T4, T5 and T6 which are disposed in a clockwise direction. Further, the coil sections Ub2, Vb2 and Wb2 are, with the concentrated windings, wound on the teeth T10, T11 and T12 which are disposed in the clockwise direction being diagonal to the teeth T4, T5 and T6 with respect to the rotor 12R.
Then, the coil sections Ua1 and Ub1, Va1 and Vb1 and Wa1 and Wb1 of the phase coils L1u, L1v and L1w, and the coil sections Ua2 and Ub2, Va2 and Vb2 and Wa2 and Wb2 of the phase coils L2u, L2v and L2w are wound on the slots SL which sandwich the respective teeth T so that the current directions are the same direction.
As stated above, the coil sections Ua1 and Ub1, Val and Vb1 and Wa1 and Wb1 of the phase coils L1u to L1w which form the three-phase motor windings L1 of the first system, and the coil sections Ua2 and Ub2, Va2 and Vb2 and Wa2 and Wb2 of the phase coils L2u to L2w which form the three-phase motor windings L2 of the second system are wound on the twelve teeth T which are different each other. That is, on the twelve teeth T, the phase coils Ua1, Va1 and Wa1 which form one of the first system are sequentially wound with the same winding direction in the clockwise direction, next the phase coils Ua2, Va2 and Wa2 which form one of the second system are sequentially wound with the same winding direction in the clockwise direction, further the phase coils Ub1, Vb1 and Wb1 which form the other of the first system are sequentially wound with the same winding direction in the clockwise direction, and finally the phase coils Ub2, Vb2 and Wb2 which form the other of the second system are sequentially wound with the same winding direction in the clockwise direction. Consequently, the coil sections for the same phase of the motor windings L1 of the first system and the motor windings L2 of the second system are wound so as not to interlink at the same time to the same magnetic flux formed due to the magnetic poles of the permanent magnets PM for the rotor 12R. Therefore, the coil sections of the three-phase motor windings L1 of the first system and the coil sections of the three-phase motor windings L2 of the second system form a magnetic circuit to suppress a magnetic interference in minimum.
In the motor control unit with the motor having the above multi-system motor windings, the motor winding is a wiring as shown in FIG. 5A at a normal operating time. However, when a primary failure (for example, a short-circuit failure of the winding) occurs, the wiring becomes to FIG. 5B, and when a secondary failure (for example, short-circuit failures of the two windings), the wirings become to FIG. 5C. Although FIGS. 5A to 5C show a case of the U-phase of the first system, the failure of other phases of other system is similarly capable of applying.
Countermeasure means to detect the failure of the above motor winding or the inverter is disclosed, for example, in Japanese Unexamined Patent Publication No. 2013-38950 A (Patent Document 1). That is, the apparatus disclosed in Patent Document 1 is to detect the failure of the inverter or the winding pair by using only the detected phase current value, and two failure judging means calculate a phase current estimating value of the own system based on a detected three-phase current value of the other system each other and detect the failure of the inverter or the winding pair by comparing the detected current value and the phase current estimating value.